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GENETICS OF MILK & TYPE

1 There have been few genetic studies on dairy goats.The principles of their genetic improvement should differ little, however, from other farm animals. Most of the genetic data available on economic traits pertain to milk and fat yields and fat percentage. They indicate differences between animals within and between breeds that can be utilized for breed improvement.
2 Breed Comparisons
The six recognized breeds of dairy goats in the United States are the Alpine, LaMancha, Nubian, Oberhasli, Saanen, and Toggenburg. According to the records of the national Dairy Herd Improvement program (DHI), production averages during 1968 to 1978 were similar for Saanen, Alpine, and Toggenburg 305-day lactations, but LaMancha and Nubian had lower milk yields in that order. Average fat percentages in Alpine, Saanen, and Toggenburg milk were also similar. The Nubian had the highest fat percentage in milk. The higher yielding breeds had lower fat contents in their milk and vice-versa. Thus, the breeds did not differ greatly in total yield of fat, but Nubian, despite their low milk yield, had the highest yield of milk fat. Oberhasli are a more recently recognized breed in the United States and insufficient records exist for breed comparison. They are expected to resemble the other Swiss breeds in milk and fat production. The average production figures in Table 1 cover a 10-year period, and little change in milk or fat production in any of the breeds seems to have occurred during that period.
3 The breed comparisons are national averages, but regional differences exist. For example, Toggenburg in the North Central and Northeast regions have higher milk and fat yields than Alpine and Saanen, but in the West, Toggenburg have lower fat yields than other breeds. The possible reasons in climate or management are not certain.
4 There is little published information on breed comparisons for body type traits. Alpine, Nubian and Saanen have similar minimum height and weight standards; LaMancha are less and Toggenburg are smallest. Considering that the average milk yield of Toggenburg does is similar to the other breeds, it is likely that Toggenburg have a high dairy merit efficiency in terms of milk yield per unit of body weight or per unit of metabolic weight (W O.75).
5 The six breeds differ in physical characteristics, particularly with respect to ear shape and face profile. Alpine, Oberhasli, and Toggenburg are of Swiss origin and have ears of medium length that are held in an upright position. LaMancha ears are externally almost absent with a ''gopher'' rudiment considered ideal. Nubian are of Asian-African origin and have long, pendulous ears. They also are characterized by a less lean body condition and a distinctly convex face profile or ''Roman'' nose. The Swiss breeds have either flat or dish-shaped face profiles.
6 Inheritance of Production and Type Traits The inheritance of milk production and most body type traits is complex. Unlike some physical traits that are simply inherited (qualitative traits) and are controlled by a few genes (for example, coat color and the presence of horns), milk production and body type traits are under the control of many genes (quantitative traits), perhaps a thousand or more. Although the individual influence of each of these genes may be small, their collective influence can be great.
7 Production and type traits also are affected by environmental factors such as feeding, management, and disease and in most cases their influence is greater than inheritance. However, this does not mean that genetic selection is not effective for improving many of these traits. Environmental influences, however, can mask genetic differences when selecting between animals, and the genetic constitution of an animal cannot be determined with certainty by physical observations or by test matings as is the case with some simply inherited qualitative traits. One cannot be sure if above or below average performance of an individual goat is due to genotype or environment. Accordingly, parents may not ''breed true'' for production and type traits, and more likely will have wide variations in their offspring. Each kid receives only a sample half of each parent's genes, which by chance can be above or below average. Furthermore, each kid can be subjected to different environmental conditions.
8 Environment Corrections Environmental influences can be controlled and corrected to permit more accurate identification of genetic differences between individual goats. Major environmental factors include level of herd management, year, age, season, parity of kidding, length of lactation, dry period and previous lactation.
9 Milk production records are expressed as deviations from the herd average (including herd level adjustments) to remove the influence of herd management and to facilitate comparisons between does from different herds. Most differences between herds for average milk production are due to feeding, housing, diseases, etc. Probably 10 to 200f the differences in production between herds are due to genetics. The remainder is due to environment.
10 One difficulty with deviations from herdmate averages arises from the small herds in which goats usually are kept. A herdmate average based on few records is less reliable than one from large numbers of herdmates. Production records vary from herd to herd, and from year to year. Therefore, they are expressed as deviations from average production of contemporaries in the same herd and year. Such herdyear deviated records facilitate genetic comparisons between animals.
11 Season of kidding has a marked influence on milk production. Does kidding between December and March have, on the average, higher milk and fat yields than does kidding later in the year. The influence of season of kidding on percentage of fat is considerably less than on yields of fat and milk, but does kidding in April to July have slightly higher fat tests than does kidding earlier. If comparisons are made between does with records initiated during different seasons, they should be adjusted for season of kidding. Season adjustment factors in conjunction with correction for age of kidding have been published for each breed.
12 Age of kidding affects milk production strongly. Age and season of kidding jointly account for 30 to 400f the total variation within a goat herd for milk and fat yields, which increase up to 5 years of age and then decline with advancing age. Effects of age on fat percentage are less pronounced.
13 Parity or lactation number, in addition to the effect of age, has a large influence on milk and fat yield in the dairy goat, which makes the dairy goat different from the dairy cow. The average difference in 305-day milk yields between first and second goat lactations is approximately 300 lb. The magnitude of this difference varies with season and region, being greatest in herds in the Western states during December to March. Factors that account for the variation of parity include the effects of previous lactation, and seasonal breeding.
14 Length of lactation also needs to be standardized when comparing dairy goat records in order not to give unfair advantage to does with abnormally long lactations. Credit for milk production is given normally up to the 305th day of lactation. This has been the traditional method for standardizing milk production records in the dairy cow, but unlike her, the goat is a seasonal breeder and the methods of standardizing need further studies. Seasonal breeding can influence the length of lactation, and this affects yields of lactations usually on the latter part of lactation. Frequency of milking influences lactation yields also and records are corrected to a two-times a day milking standard.
15 The following example illustrates how adjustments for known environmental factors can be made to enable comparisons between goats on the basis of their milk production records. Consider two Alpine does with 305-day milk yield records: Daisy with 2000 lb and Buttercup with 1400 lb (Table 2). At a first glance, one might think that Daisy is the better doe. However, she initiated her lactation in January at 24 months of age while Buttercup began in April at 12 months of age. Buttercup kidded at a younger age and less favorable season. The age-season adjustments correct for this handicap. Her records then are calculated as deviations from the average milk yield of other Alpine does with their records adjusted for age, season of kidding, and same herd and year as Daisy and Buttercup. As a result Buttercup's production is 258 lb above herd-year-season average and Daisy's is 70 lb below. Given no other information and assuming that the herdmates of each doe were genetically similar, one should select Buttercup over Daisy as the better doe for milk yield.
16 As far as body type traits of dairy goats are concerned, there is little published information on the effects of environment, although its factors certainly have an influence; for example, stage of lactation causes some conformational changes, particularly udder characteristics. Research is needed to improve the ability to select goats for these traits. Type classification programs initiated in recent years for goats may provide important data soon.
17 Heritability indicates the relative importance of heredity in the expression of a trait and measures the average percentage of variation between individuals that is due to heredity. For example; 300f differences between individuals for a particular trait are attributable to genetics but the remaining 70 are due to environmental influences if that trait had a heritability of 30
18 Heritability also gives an indication of the potential for phenotypic selection to improve a trait genetically. A high heritability suggests that individual selection will produce rapid genetic improvement, but a low heritability value indicates that progress from such selection will be slow and other means should be adopted to improve the trait.
19 The heritability of milk yield of goats is moderate and about 30 Fat yield has a similar heritability. Heritability of fat percentage is higher about 50, and although few data exists,heritabilities of other milk constituents such as protein and solids-not-fat are similar to that of fat percentage.
20 Applying the concept of heritability to the genetic evaluation of goats (Table 2), one expects Buttercup to be 77 lb above average (30 of +258) and Daisy to be 21 lb below average (300f -70) in genetic merit for milk yield. The remaining 700f the deviation from herdmates (Table 2) would be due to unexplained environmental influences.
21 Data on heritability of body traits of dairy goats are scarce. There have been numerous studies on heritability of type of dairy cattle, and they indicate that most conformation traits have moderate to low heritabilities. It is likely that this applies also to dairy goats.
22 Repeatability Performance of goats either good or poor, tends to be repeated although not perfectly. Repeatability represents the average correlation among records of the same animal and measures the tendency to be similar on successive records by the same animal. The repeatability of successive milk and fat yield records of the same doe is about 50 Repeatability of fat percentage, and probably the other major milk constituents, is approximately 60 Few repeatability estimates of the type traits are available.
23 Repeatability can be used to estimate future production of a doe on the basis of her past performance using the formula for the Probable Producing Ability (PPA): nr (Average adjusted deviation from herdmates) 1 + (n-1) r where n is the number of records, and r is repeatability of the trait. For example, the estimated future probable producing ability for milk yield of a doe with 3 lactation records averaging 200 lb above herdmates is +150 lb (assuming r = 0.50; 0.75 x 200 = 150). Not all does will produce as expected, but the fewest errors in estimation will be made when this method is used.
24 Repeatability (r) and heritability (hh) in a formula for the Estimated Breeding Value (EBV) can be used to estimate the genetic value of a doe on her past performance: nhh (Average adjusted deviation from herdmates) 1 + (n-1) r Assuming hh = 0.30 and r = 0.50 for milk yield, the EBV of the doe from the previous example is +90 lb (0.45 x 200 = 90). This procedure can be used to rank does conveniently for genetic value although they differ in number of lactations.
25 Improvement of Several Traits
Selection for one trait, seldom leaves other traits of economic importance undisturbed. Many production and type traits are correlated genetically; i.e. they are influenced by some of the same genes.
26 Milk and fat yield of goats have strong positive genetic correlations so that selection for increased milk yield results in increased fat yield also, and vice-versa. In contrast, milk yield and fat percent age are correlated negatively. Selection for milk yield results in some correlated depression in fat percentage. Similarly, selection for fat percentage will result in a correlated decrease in milk yield.
27 Within breed, increased body weight and stature are related genetically with increased milk production. Body size accounts for approximately 10 to 150f the variation among does in milk yield. Therefore, selection for milk yield results in some increase in doe size.
28 The primary purpose of the dairy goat is to produce high quality milk efficiently. Accordingly, milk production must be a major selection goal. The more traits that are selected in a breeding program, the less the progress that can be made for any single trait. Therefore, the breeder must restrict his selection emphasis for traits other than milk yield to only those that have economic importance and large heritabilities to respond effectively to selection.